Printed circuit boards, known commonly as PCBs, are ubiquitous in the electronics industry. In addition to connecting integrated circuits (ICs) and other components, the PCB also is a mechanical mounting surface for the devices. A multi-layered PCB typically includes at least one signal layer and reference potential layers, also known as ground and power.
The PCB consists of a laminate of a conductive material, usually copper, as well as an insulating dielectric substrate. The traces formed of the copper material provide signaling paths for communication between the ICs, discrete components, or other circuitry mounted on the PCB. Under certain operating conditions, the traces act like transmission lines. In particular, board designs involving high-speed signals pay close attention to the impedance of these communication paths.
Impedance is a measure of passive opposition to the flow of current along the trace. Impedance consists of resistance (to direct current), reactance (to alternating current), inductance and capacitance. The length and width of each trace, its proximity to other traces, and the number of board layers are among the many factors affecting trace impedance. Generally, wider traces have lower impedances, where other factors are equal.
An impedance mismatch is the discontinuity between the impedances of two communicating components. When an impedance mismatch is present, reflection along the signal trace can occur. The reflected signal will add to or subtract from the original signal being transmitted between the components, causing a distortion and, possibly, a failure of the transmission.
Generally, PCB boards are manufactured to meet certain trace impedances, within some tolerance. Thus, where a 50-ohm PCB is specified, plus or minus fifteen percent, the impedance of all traces on the PCB will be between 42.5 and 57.5 ohms. A single PCB can simultaneously include traces of different widths, such as 50-ohm traces and 60-ohm traces, for example. This enables signal groups with different impedance requirements to simultaneously occupy the PCB. A memory interface may have a 60-ohm impedance requirement while a processor interface, located on the same PCB, has a 50-ohm impedance requirement.
Despite having a board with a known trace impedance, board designers ensure that the impedance between devices matches whenever possible. Impedance matching improves signal integrity by reducing reflections and ringing along the trace that may adversely affect system performance.
Most traces on the PCB are terminated, such as by adding resistors or buffers to the output and/or the input of an integrated circuit. The output impedance or input termination of the circuit is matched to the characteristic impedance of the connecting trace. This impedance matching is a consideration for most system designs.
Many system designs include impedance compensation circuitry, such as resistive compensation or controlled impedance drivers. Impedance compensation circuitry may be dedicated to separate signal groups of a system design. Thus, a memory controller may have its own impedance compensation circuitry, likewise for the processor, the I/O controller, and so on. A single PCB may thus maintain several impedance compensation circuits for various interfaces.
Studies have shown that the variation of trace impedance between two locations on the same layer of the PCB is lower than when the two locations are on different layers of the PCB. In particular, same layer variation in the trace impedance on some PCBs may be about five percent. The trace impedance variation between a first location on a first layer and a second location on a second layer (microstrip) is approximately seven to ten percent.
While impedance compensation circuitry facilitates impedance matching, it fails to consider the differences in trace impedances between same-layer and different-layer traces. Thus, there is a need to develop impedance compensation circuitry that accounts for the known variations in trace impedances.